From: Freshairconsult@aol.com 

To: Brenda Shine/RTP/USEPA/US@EPA 

Cc: Chittim@api.org 

Date: 11/24/2009 05:18 PM 

Subject: Additional Detail on Flare Seal Drum Monitoring 

Brenda: 

 

Here is some additional detail on the flare drum seal monitoring approach, that API believes should be 

allowed as an option, to supplement the flare flow monitoring required under Subpart Ja. It represents my 

understanding from discussions with API and WSPA members. Let me know if you have any questions. 

 

Have a great Thanksgiving. 

 

Norm 

 

Norman Morrow, Ph.D. 

Fresh Air Consulting 

18126 Spellbrook Drive 

Houston, TX 77084 

(281) 813-8633 

freshairconsult@aol.com 

(See attached file: Flare Seal Drum Monitoring Supplement 11-24-09.docx) 
November 24, 2009 

Flare Water Seal Monitoring Specifics 

In order to identify if low flare gas flow meter readings are real in flare systems with water seals, 

it has often proved valuable to monitor the seal integrity as a check on the flare gas flow meter. 

As long as the flare water seal remains intact, there will be no gas flow to the flare and any flare 

gas flow meter reading is assumed to be incorrect or to reflect flow in the flare header associated 

with changing pressures as flare gas recovery systems operate. This approach has been accepted 

by both the South Coast Air Quality Management District and the Bay Area Air Quality 

Management District. 

Typically, the water level in a flare seal drum is maintained using an overflow weir inside the 

drum or in drums with only one internal compartment an overflow drain from the drum, though 

in some cases the level is maintained using a level controller. The amount of seal is set by 

placing the downward pointing inlet flare line at the desired distance below the water level. 

Depending on the system design, make-up water is supplied continuously or as needed or both to 

maintain the level. Flare water seal integrity may be confirmed by continuously monitoring the 

water level along with either the flare gas header pressure or the pressure drop across the seal 

drum. In some configurations, water level in the seal drum will vary as the flare header pressure, 

flare stack backpressure and water makeup varies. In monitoring for seal integrity, the primary 

purpose of the level instrument is to assure that the water level has not varied significantly from 

the design value, but in situations where the water level does vary significantly, the level 

instrument may be used to determine the drum upstream pressure or pressure drop that indicates 

flow to the flare at any given time. 

Water seal height depends on the size of the drum, the size of the flare header, and the 

characteristics of the gas streams that are potentially routed to the flare. Thus, the backpressure 

the seal exerts on the flare header will vary from drum to drum. Typically, this backpressure is 

in the range of 0.1 to 1.0 psi, corresponding to roughly a 3” to 30” water seal. For systems where 

the flare stack exerts negligible and/or constant, known backpressure on the seal drum, a pressure 

monitor in the flare header upstream of the water seal can be used to indicate if the design 

backpressure has or has not been exceeded. Because flare headers are typically large diameter 

and, for safety reasons, are designed without low points that could form liquid seals, the 

upstream pressure can be measured anywhere in the flare header between the last gas entry point 

and the seal drum. For systems where the flare stack has a significant backpressure or a variable 

backpressure, the seal drum delta pressure may be monitored. Flare stack backpressure depends 

on the type of flare (i.e., elevated or ground), specifics of the flare stack design and combustion 

assist system (i.e., air, steam), if any, presence and impact of purge gas on the flare stack 

pressure drop and other factors. 

Typical process type pressure and level instruments are used for this monitoring and, as already 

done throughout the NSPS rules (including NSPS Ja) they are simply specified in the rules as 

continuous monitors. For instance, §60.105a of Subpart Ja specifies parameter monitoring for 

FCCU controls using CPMS as follows: 
November 24, 2009 

(b) Control device operating parameters . Each owner or operator of a FCCU or FCU 

subject to the PM per coke burn-off emissions limit in §60.102a(b)(1) shall comply with 

the requirements in paragraphs (b)(1) through (3) of this section. 

(1) The owner or operator shall install, operate, and maintain continuous parameter 

monitor systems (CPMS) to measure and record operating parameters for each control 

device according to the requirements in paragraph (b)(1)(i) through (iii) of this section. 

(i) For units controlled using an electrostatic precipitator, the owner or operator shall 

use CPMS to measure and record the hourly average total power input and secondary 

voltage to the entire system. 

(ii) For units controlled using a wet scrubber, the owner or operator shall use CPMS to 

measure and record the hourly average pressure drop, liquid feed rate, and exhaust gas 

flow rate. As an alternative to a CPMS, the owner or operator must comply with the 

requirements in either paragraph (b)(1)(ii)(A) or (B) of this section. 

(A) As an alterative to pressure drop, the owner or operator of a jet ejector type wet 

scrubber or other type of wet scrubber equipped with atomizing spray nozzles must 

conduct a daily check of the air or water pressure to the spray nozzles and record the 

results of each check. 

(B) As an alternative to exhaust gas flow rate, the owner or operator shall comply with 

the approved alternative for monitoring exhaust gas flow rate in 40 CFR 63.1573(a) of 

the National Emission Standards for Hazardous Air Pollutants for Petroleum Refineries: 

Catalytic Cracking Units, Catalytic Reforming Units, and Sulfur Recovery Units. 

(iii) The owner or operator shall install, operate, and maintain each CPMS according to 

the manufacturer's specifications and requirements. 

Thus for flare water seal monitoring, the requirement would be to use continuous parameter 

monitors for pressure (or delta pressure) and water level, as applicable. 

In addition to whether the upstream pressure or seal drum delta pressure are monitored and the 

location of the monitors, the pressure and level values that indicate the water seal has been lost 

are specific to every individual flare seal drum. Therefore, it is recommended that, where a 

source chooses to use this supplemental monitoring, they include the specifics of the monitoring 

and the process values that indicate seal loss in the flare management plan required by 

§60.103a(a). 
From: Brenda Shine/RTP/USEPA/US 

To: Freshairconsult@aol.com 

Date: 11/24/2009 10:41 AM 

Subject: Re: NSPS Ja 

Thanks Norm. 

Yes, I 'm still concerned that we don't have the actual method well specified. My understanding is that its 

a continuous pressure measurement of the waste gas and a comparison against the pressure exerted by 

the water seal (which is measured by height). If the pressure ever exceeds the pressure from the water 

seal then a release has occurred. 

The devil is in the details, however. 

I need details to understand: 

- where the pressure monitoring occurs for the waste gas, 

- what frequency (it would seem to have to be continuous, but in our rules continuous is often every 15 

minutes, which wouldn't seem adequate) 

- what accuracy/precision on the pressure monitor 

- how the pressure exerted by the water seal is measured 

- what frequency 

- what accuracy/precision on the water seal measurement 

Also, possibly a qualitative description on what systems this can work on and what systems it can't. 

Brenda Shine 

Environmental Engineer 

US Environmental Protection Agency 

Research Triangle Park, NC 27711 

(919) 541-3608, Mail Drop E143-01 

From: Freshairconsult@aol.com 

To: Brenda Shine/RTP/USEPA/US@EPA 

Date: 11/20/2009 04:06 PM 

Subject: NSPS Ja 

Brenda: 

Here's electronic copies of yesterdays handouts. It was a good discussion. Thanks for your attention. 

Doug Van Pelt tells me you could some details on flare water seal monitoring. I will round those up and 

send that info to you as soon as I can, probably Monday or Tuesday. 

Norm